1. Field of the Invention
The present invention relates to a process for the dissociation of methyl tertiary butyl ether (MTBE) to produce isobutene.
2. Related Art
The reaction to produce MTBE is known to be reversible, i.e., the MTBE will dissociate to produce methanol and isobutene which were originally combined to produce the MTBE. For example, U.S. Pat. Nos. 3,121,124; 3,170,000; 3,634,534; 3,634,535; 4,232,177 and 4,320,232 disclose the dissociation alkyltertiary alkyl ethers using ion exchange resin catalysts.
MTBE is of economic importance because of its use as an octane improver in unleaded gasoline. However, there may be a time when MTBE will be in over capacity or its cost very low because of large numbers of producers. One interesting aspect of MTBE production is the selective nature of the reaction of methanol with the isobutene as compared to the other components in a C.sub.4 stream. That is, the isobutene may be substantially removed from a C.sub.4 stream by contacting the stream with methanol in the presence of a suitable acid catalyst (most processes use acidic cation exchange resins). The separation of the MTBE from the unreacted components of the C.sub.4 stream is easily effected by distillation and a high purity MTBE product is produced.
Hence the dissociation of MTBE will produce a stream containing substantially only isobutene, as the olefin, methanol; and some oxygenated compounds and polymer impurities. The dissociation of MTBE using a suitable catalyst is a relatively clean process for obtaining an isobutene stream, compared to the cold acid treatment by which high purity isobutene is currently obtained. Thus, with the increasing demand for isobutene, the dissociation of MTBE for isobutene is viable and economic. Furthermore, the cold acid process not only requires large amounts of energy, but is highly corrosive because of the sulfuric acid used. The use of cationic resin catalysts or other catalyst such as phosphoric acid supported on silica gel, alumina, supported metal sulfates for the dissociation require less energy and are substantially free of corrosion.
The dissociation reaction is favored by higher temperatures than the reaction of methanol and isobutene. The former reaction is endothermic, whereas the latter reaction is exothermic. The acid cation exchange resins are somewhat more active than other catalyst, hence somewhat lower temperatures are required for the dissociation, thus less energy input is required and operation at lower pressures is possible. Those acidic cation exchange resins used heretofore have initially exhibited good selectivities and conversion, however, as the reaction continues and higher temperatures are required to obtain the optimum results, degradation of these catalysts results. The degradation has been both physical, e.g., melting, and chemical, e.g., loss of the sulfuric groups which are incorporated in these resins to form acid sites. Furthermore, operating a system to maximize the dissociation of the MTBE, when the catalyst is rapidly degrading, requires greater energy, higher temperatures and loss in selectivity to isobutene.
The use of acid cation exchange resins in the past for the dissociation of methyl tertiary butyl ether has been demonstrated, i.e., U.S. Pat. No. 3,121,124 (Verdol) using a gel type catalyst (Dowex 50) and U.S. Pat. No. 4,232,177 (Smith) used a macroreticular catalyst (Amberlyst 15) in a process designated as catalytic distillation.
Both of these catalysts exhibit instability at the higher temperatures required for dissociation, i.e., the sulfonic groups are lost from the resin. This effect becomes more pronounced and accelerated with the increases in temperature which are required as the catalysts age. Moreover, the dissociation of the ether is an endothermic reaction which is favored by the absence of a liquid phase, that is, an entirely vapor process such as Verdol used is preferred for the dissociation as compared to the Smith process which is partially in the liquid phase because of the nature of distillation, i.e., a distillation contains both vapor phase (the distillate) and a liquid phase (the internal downflow).
Since the vapor phase favors the dissociation reaction, many dissociation processes employ catalyst which can operate at higher temperatures without damage, however, these catalysts would appear not to be as selective as the cation exchange resin catalysts, nor quite as active. Also since the reaction is endothermic, catalysts which operate at higher temperatures require a greater input of energy into the system, which on an industrial scale may make them uneconomic.
It has been found that stablized cation exchange resins, i.e., stabilized and designed to operate at higher temperatures, actually are more active at lower temperatures than prior unstable catalysts and the thermal stability of the catalyst in use exhibit a favorable time trend.
It is an advantage of the present invention that high selectivity to isobutene production is obtained at reasonable conversion rates, using moderate temperatures. It is a feature of this invention that the unconverted MTBE may be recycled to the reaction for the synthesis of MTBE.